A limiting factor in the performance of an internal combustion engine is the amount of combustion air that can be delivered to the intake manifold for combustion in the engine cylinders. Atmospheric pressure is often inadequate to supply the required amount of air for proper operation of the engine. An internal combustion engine, therefore, may include one or more turbochargers for compressing air to be supplied to the combustion chambers provided within corresponding combustion cylinders. The turbocharger supplies combustion air at a higher pressure and density than existing atmospheric pressure and ambient density. The use of a turbocharger can compensate for lack of power due, for example, to altitude, or to otherwise increase power that can be obtained from an engine of a given displacement, thereby reducing cost, weight, and size of the engine required for a given output.
A turbocharger typically includes a turbine driven by exhaust gases from the engine, and a compressor driven by the turbine. The compressor receives from atmosphere the air to be compressed and supplies the air to the combustion chamber. A common shaft interconnects the turbine wheel of the turbine with the compressor wheel in the compressor section. A stream of exhaust gases from the engine is conducted from the exhaust manifold to the turbine. The stream of exhaust gases passing through the turbine causes the turbine wheel to rotate, thereby turning the common shaft and rotating the compressor wheel.
Several problems are experienced with previously known constructions for turbochargers. For instance, turbochargers generally take some time to gain speed and provide increased pressure when increased power demands are placed on the system. This generally is the result of rotational inertia of the turbocharger. Therefore, when the engine is operating under conditions that require quick increases in power, a delay period occurs while the turbocharger accelerates and desired instantaneous increases in power cannot be achieved. This also holds true when the engine is operating under conditions that require quick decreases in power and pressure. The addition and removal of large electrical loads, or “block loads”, incurred while providing a constant rotational speed for the engine are examples of transient loads that may also require quick changes in power and therefore corresponding changes in turbocharger operating speed.
As the engine runs, the turbocharger will cycle through various ranges of rotational speed as power is added or removed. Because of the rotational inertia of the turbocharger, it is often necessary to run the engine at low efficiency to control heating of the exhaust flow during periods of transient loads, thereby controlling rotational speeds of the turbocharger. These low efficiency operations have the drawback of increasing emissions of, for example, soot and nitrogen dioxides.
One solution for improving response of the turbocharger is described in U.S. Patent Application Publication No. 2004/005506 to Shaffer, entitled “Inertia Augmented Turbocharger.” The Shaffer document describes a turbocharger having at least one flywheel configured for releasable coupling to the shaft of the turbocharger. More specifically, a first flywheel of Shaffer is positioned between the turbine and the compressor and may directly engage, via a first clutch, the turbocharger shaft. The first flywheel is used primarily to add energy to the turbocharger shaft, and is maintained at an operating rotational speed by selectively directing an air flow over vanes formed around the first flywheel. A second flywheel is also positioned between the turbine and the compressor and may directly engage, via a second clutch, the turbocharger shaft. The second flywheel is used primarily to remove energy from the turbocharger shaft, and therefore is normally stationary until needed. A controller operates the first clutch to permit the first flywheel to add power to the turbocharger shaft and the second clutch to permit the second flywheel to remove power from the turbocharger shaft.
While the arrangement disclosed in Shaffer improves response time of the turbocharger, it also has some drawbacks. By placing the flywheels in a high temperature location between the compressor and the turbine, the capacity of the flywheels is limited. That is, the elevated temperatures within the turbocharger housing restrict the use of certain flywheel materials, such as carbon fiber, that have a higher strength-to-weight ratio and can rotate at higher speeds (such as, for example, approximately 60,000 rpm or more), and therefore have a greater capacity to store and release energy, but are otherwise more sensitive to elevated temperatures. Slight increases in operating temperature may quickly lead to potentially catastrophic damage of the flywheel, as carbon fiber material may delaminate and disintegrate when operating temperatures exceed approximately 170° C. The increased temperature may also increase the pressure surrounding the flywheel, which may degrade flywheel performance by increasing friction forces acting on the flywheel. The Shaffer flywheel arrangement also fails to make productive use of energy removed from the turbocharger shaft.